Fingerprint sensing circuit, electronic device and method for processing fingerprint image

ABSTRACT

The fingerprint sensing circuit includes sensing units and a sensing circuit. The sensing units are arranged as columns and rows. Each of the sensing units includes a sensing electrode. The sensing circuit is electrically connected to the sensing electrodes in at least two first sensing units of the sensing units in a first period to obtain a first fingerprint grey level.

BACKGROUND Field of Invention

The present invention relates to a fingerprint sensing circuit, anelectronic device and a method for processing a fingerprint image.

Description of Related Art

Human fingerprints are detailed, nearly unique, difficult to alter, anddurable over the life of an individual, making them suitable aslong-term markers of human identity. In general, a fingerprint sensingcircuit includes multiple sensing electrodes arranged as a matrix. Afingerprint image is generated by the capacitance difference between thesensing electrodes and a human finger for identifying ridges and valleyson the human finger. One or ore insulation layer is disposed between thesensing electrodes and the finger, and when the insulation layer is verythick, it's hard to identify the ridges and the valleys. On the otherhand, the identification ability is improved when the area of thesensing electrode is increased, but it will also increase the area ofthe circuit or reduce the resolution of the fingerprint image. It is anissue in the art about how to provide better fingerprint image under thehardware limitations.

SUMMARY

Embodiments of the present invention provide a fingerprint sensingcircuit including sensing units and a sensing circuit. The sensing unitsare arranged as columns and rows. Each of the sensing units includes asensing electrode. The sensing circuit is electrically connected to thesensing electrodes in at least two first sensing units of the sensingunits in a first period to obtain a first fingerprint grey level.

In some embodiments, after obtaining the first fingerprint grey level,the sensing circuit is electrically connected to the sensing electrodesin at least two second sensing units of the sensing units in a secondperiod to obtain a second fingerprint grey level. Part of the secondsensing units is identical to part of the first sensing units.

In some embodiments, the number of the first sensing units is equal to4, and the number of the second sensing units is equal to 4. The firstsensing units and the second sensing units are arranged in a first rowand a second row of the rows. The first sensing units are arranged in afirst column and a second column of the columns. The second sensingunits are arranged in the second column and a third column of thecolumns.

In some embodiments, each of the sensing units further includes atransfer switch having a first terminal, a second terminal and a controlterminal. The first terminal of the transfer switch is connected to therespective sensing electrode. The fingerprint sensing circuit furtherincludes first conductive lines and second conductive lines. The firstconductive lines are respectively corresponding to the rows. Each of thefirst conductive lines is electrically connected to the controlterminals of the transfer switches in one of the rows. The secondconductive lines are respectively corresponding to the columns andelectrically connected to the sensing circuit. Each of the secondconductive lines is electrically connected to the second terminals ofthe transfer switches in one of the columns.

In some embodiments, the fingerprint sensing circuit further includescolumn switches, a row select circuit and a column select circuit. Thecolumn switches are respectively disposed on the second conductive linesand disposed between the sensing units and the sensing circuit. The rowselect circuit transmits a first signal to the first conductive linescorresponding to the first row and the second row in the first periodfor turning on the transfer switches in the first row and the secondrow. The column select circuit turns on the column switchescorresponding to the first column and the second column in the firstperiod.

In some embodiments, the row select circuit transmits the first signalto the first conductive lines corresponding to the first row and thesecond row in the second period to turn on the transfer switches in thefirst row and the second row. The column select circuit turns on thecolumn switches corresponding to the second column and the third columnin the second period.

From another aspect, embodiments of the present invention provide amethod for processing a fingerprint image for an electronic deviceincluding a fingerprint sensing circuit. The fingerprint sensing circuitincludes sensing units, and each of the sensing units includes a sensingelectrode. The method includes: receiving fingerprint grey levels fromthe fingerprint sensing circuit, in which the fingerprint grey levelsare respectively corresponding to the sensing units; and performingweighting summation on first fingerprint grey levels of the fingerprintgrey levels to obtain a first new fingerprint grey level to replace oneof the first fingerprint grey levels.

In some embodiments, the method further includes: performing theweighting summation on second fingerprint grey levels of the fingerprintgrey levels to obtain a second new fingerprint grey level to replace oneof the second fingerprint grey levels, in which part of the firstfingerprint grey levels is identical to part of the second fingerprintgrey levels.

In some embodiments, the number of the first fingerprint grey levels isequal to 4, the number of the second fingerprint grey levels is equal to4, and two of the first fingerprint grey levels are identical to two ofthe second fingerprint grey levels respectively.

In some embodiments, the sensing units are arranged as columns and rows,the first fingerprint grey levels are corresponding to first sensingunits of the sensing units, and the second fingerprint grey levels arecorresponding to second sensing units of the sensing units. The firstsensing units and the second sensing units are arranged in a first rowand a second row of the rows. The first sensing units are arranged in afirst column and a second column of the columns. The second sensingunits are arranged in the second column and a third column of thecolumns.

From another aspect, embodiments of the present invention provide anelectronic device including a fingerprint sensing circuit. Thefingerprint sensing circuit includes sensing units and a sensingcircuit. The sensing units are arranged as columns and rows, and each ofthe sensing units includes a sensing electrode. The sensing circuit iselectrically connected to the sensing electrodes in at least two firstsensing units of the sensing units in a first period to obtain a firstfingerprint grey level.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood by reading the followingdetailed description of the embodiment, with reference made to theaccompanying drawings as follows.

FIG. 1 is a schematic diagram illustrating capacitive print sensingaccording to an embodiment.

FIG. 2 is a schematic diagram illustrating a fingerprint sensing circuitaccording to an embodiment.

FIG. 3 is a schematic diagram illustrating an electronic deviceaccording to an embodiment.

FIG. 4 is a schematic diagram illustrating the fingerprint according toan embodiment.

FIG. 5 is a diagram illustrating the processed fingerprint imageaccording to an embodiment.

FIG. 6 is a diagram illustrating a flowchart of a method for processingfingerprint image.

DETAILED DESCRIPTION

Specific embodiments of the present invention are further described indetail below with reference to the accompanying drawings, however, theembodiments described are not intended to limit the present inventionand it is not intended for the description of operation to limit theorder of implementation. Moreover, any device with equivalent functionsthat is produced from a structure formed by a recombination of elementsshall fall within the scope of the present invention. Additionally, thedrawings are only illustrative and are not drawn to actual size.

The using of “first”, “second”, “third”, etc. in the specificationshould be understood for identifying units or data described by the sameterminology but are not referred to particular order or sequence.

FIG. 1 is a schematic diagram illustrating capacitive fingerprintsensing according to an embodiment. Referring to FIG. 1, a human fingerhas a ridge 101 and a valley 102. A top surface of a protection layer110 (may be referred to an insulation layer) faces the user, and sensingelectrodes 111 and 112 are disposed on a bottom surface of theprotection layer 110. When the finger touches the protection layer 110,the distance between the ridge 101 and the top surface of the protectionlayer 110 is relatively shorter, and the distance between the valley 102and the top surface of the protection layer 110 is relatively longer. Acapacitor C_(i) is formed between the ridge 101 and the sensingelectrode 111, and capacitors C_(i) and C_(a) connected in series areformed between the valley 102 and the sensing electrode 112. Inaddition, capacitors C_(p) are formed between the sensing electrodes111-112 and a substrate 120. Therefore, the capacitance measured on thesensing electrode 111 is C_(p)+C_(i), and the capacitance measured onthe sensing electrode 112 is C_(p)+C_(i)∥C_(a). The ridge 101 and thevalley 102 are identified by identifying the two capacitances. Ingeneral, the larger the difference between the two capacitances is, theeasier it is to identify the ridge 101 and, the valley 102. Assume theareas of the sensing electrodes 111 and 112 are the same (denoted as Abelow), the thickness of the protection layer 110 is d_(i), the distancebetween the valley 102 and the protection layer 110 is d_(a), thedielectric coefficient of the protection layer 110 is ε_(i), thedielectric coefficient of air is ε₀, and then the difference between thetwo capacitances is written in following equation (1).

$\begin{matrix}{\left. {C_{i} - C_{a}}||C_{i} \right. = {{\frac{ɛ_{i}A}{d_{i}} - \frac{ɛ_{0}ɛ_{i}A}{{d_{i}ɛ_{0}} + {d_{a}ɛ_{i\;}}}} = {ɛ_{i}{A\left( \frac{d_{a}ɛ_{i}}{d_{i}\left( {{d_{i}ɛ_{0}} + {d_{a}ɛ_{i}}} \right)} \right)}}}} & (1)\end{matrix}$

As shown in the equation (1), the larger the area A is, the larger thecapacitance difference is, and thus the identification ability getsbetter; and the larger the thickness d_(i) is, the smaller thecapacitance difference is, and thus the identification ability getsworse. A fingerprint sensing circuit is provided in the disclosure whichis capable of improving the identification ability without altering thearea A and the thickness d_(i).

FIG. 2 is a schematic diagram illustrating a fingerprint sensing circuitaccording to an embodiment. Referring to FIG. 2, a fingerprint sensingcircuit 200 includes multiple sensing units 210(1,1)-210(3,4), a rowselect circuit 220, a column select circuit 230 and a sensing circuit240. For the sake of simplification, only 12 sensing units areillustrated in FIG. 2, but the number of the sensing units included inthe fingerprint sensing circuit 200 is not limited in the invention. Thesensing units 210(1,1)-210(3,4) are arranged as 4 columns C1-C4 and 3rows R1-R3. Each of the sensing units 210(1,1)-210(3,4) includes asensing electrode SE and a transfer switch TSW (for simplification, onlysensing electrode SE and transfer switch TSW in the sensing unit210(1,1) are labeled), and the sensing electrode SW is illustrated as acapacitor in FIG. 2. Each transfer switch TSW has a first terminal, asecond terminal and a control terminal. The first terminal of thetransfer switch TSW is connected to the respective sensing electrode SE.The fingerprint sensing circuit 200 further includes multiple firstconductive lines 251-253 and second conductive line 261-264. The firstconductive lines 251-253 are respectively corresponding to the rowsR1-R3 and the second conductive lines 261-264 are respectivelycorresponding to the columns C1-C4. Each one of the first conductivelines 251-253 is electrically connected to the control terminals of thetransfer switches TSW in the corresponding row. For example, the firstconductive line 251 is electrically connected to the control terminalsof the all the transfer switches TSW in the row R1, and so on. Each oneof the second conductive lines 261-264 is electrically connected to thesecond terminals of all transfer switches TSW in the correspondingcolumn. For example, the second conductive line 261 is electricallyconnected to the second terminals of all transfer switches TSW in thecolumn C1, and so on. The first conductive lines 251-253 are connectedto the row select circuit 220. The second conductive lines 261-264 areconnected to the sensing circuit 240. Column switches CSW1-CSW4 arerespectively disposed on the second conductive lines 261-264 anddisposed between the sensing units 210(1,1)-210(3,4) and the sensingcircuit 240. The column select circuit 230 is configured to turn on/offthe column switches CSW1-CSW4.

Each one of the sensing units 210(1,1)-210(3,4) represents a pixel. Insome embodiments, the sensing circuit 240 includes an analog to digitalconverter 241 for outputting a value according to the voltage on thesensing electrode SE as a grey level (referred to a fingerprint greylevel below), in which the value reflects the capacitance on the sensingelectrode SE, and all pixels constitute a fingerprint image. In priorart, the sensing circuit 240 is electrically connected to one sensingelectrode in a sensing period to obtain one fingerprint grey level.However, in the embodiment, the sensing circuit 240 is electricallyconnected to the sensing units in at least two sensing units (alsoreferred to first sensing units) in a first period to obtain onefingerprint grey level (also referred to first fingerprint grey level).For example, in the first period, the sensing circuit 240 may beelectrically connected to four sensing units 210(1,1), 210(1,2),210(2,1) and 210(2,2) to obtain one fingerprint grey level. In someembodiments, the sensing circuit 240 may be electrically connected totwo sensing units 210(1,1) and 210(1,2) in the first period to obtainone fingerprint grey level. How many sensing units and which sensingunits that the sensing circuit 240 is electrically connected to are notlimited in the invention. In some embodiments, the sensing circuit 240senses self-capacitance from the sensing electrodes. When the sensingcircuit 240 is electrically connected to more than one sensingelectrodes, the sensing electrodes are electrically connected to theeach other and to the analog to digital converter 241, and thus thesensing electrodes are connected to the each other in parallel, and thearea of the sensing electrode is equivalently increased, and thus theidentification ability is improved.

The aforementioned first period indicates how long the generation of thefingerprint grey level is required, but the length of the first periodis not limited in the invention. In some embodiments, in a second periodafter the first period, the sensing circuit 240 is electricallyconnected to the sensing electrodes in at least two sensing units (alsoreferred to second sensing units) to obtain one fingerprint grey level(also referred to second fingerprint grey level), in which part of thesecond sensing units is identical to part of the first sensing units.For example, the first sensing units are 210(1,1), 210(1,2), 210(2,1)and 210(2,2), and the second sensing units are 210(1,2), 210(1,3),210(2,2) and 210(2,3), in which the sensing units 210(1,2) and 210(2,2)are overlapped. From another aspect, in the example above, the number ofthe first sensing units is equal to 4, the number of the second sensingunits is equal to 4. The first sensing units and the second sensingunits are arranged in the first row R1 and the second row R2, the firstsensing units are arranged in the first column C1 and the second columnC2, and the second sensing units are arranged in the second column C2and the third column C3.

In detail, in the first period, the row select circuit 220 transmits afirst signal (e.g. high-level signal) to the first conductive lines 251and 252 corresponding to the first row R1 and the second row R2 to turnon the transfer switches TSW in the first row R1 and the second row R2.In addition, the column select circuit 230 turns on the column switchesCSW1 and CSW2 corresponding to the first column C1 and the second columnC2 in the first period. Consequently, the sensing electrodes SE in thesensing units 210(1,1), 210(1,2), 210(2,1) and 210(2,2) are electricallyconnected to the sensing circuit 240, and the analog to digitalconverter 241 would output the first fingerprint grey level.

In the second period, the row select circuit 220 transmits the firstsignal to the first conductive lines 251 and 252 to turn on the transferswitches TSW in the first row R1 and the second row R2. In addition, thecolumn select circuit 230 turns on the column switches CSW2 and CSW3corresponding to the second column C2 and the third column C3.Therefore, the sensing electrodes in the sensing units 210(1,2),210(1,3), 210(2,2) and 210(2,3) are electrically connected to thesensing circuit 240, and the analog to digital converter 241 wouldoutput the second fingerprint grey level.

In some embodiments, in a third period, the sensing circuit 240 iselectrically connected to the sensing units 210(1,3), 210(1,4),210(2,3), and 210(2,4), in a fourth period, the sensing circuit 240 iselectrically connected to the sensing units 210(1,4) and 210(2,4); in afifth period, the sensing circuit 240 is electrically connected to thesensing units 210(2,1), 210(2,2), 210(3,1) and 210(3,2). Therefore, theresolution of the fingerprint image does not change while the sensingcircuit 240 uses more than one sensing units to generate one fingerprintgrey level because the sensing units used in different periods arepartially overlapped.

FIG. 3 is a schematic diagram illustrating an electronic deviceaccording to an embodiment. Referring to FIG. 3, an electronic device300 includes the aforementioned fingerprint sensing circuit 200 and alogical operation circuit 301. The electronic device 300 may be smartphone, tablet, personal computer, laptop, industry computer or any typeof mobile device, which is not limited in the invention. The logicaloperation circuit 301 may be central processing unit, microprocessor,microcontroller, digital signal processor, image processing chip orapplication-specific integrated circuit, etc. Alternatively, the logicaloperation circuit 301 may be integrated into the fingerprint sensingcircuit 200. In some embodiments, the fingerprint sensing circuit 200operates as the embodiments described above, and the obtainedfingerprint grey levels are transmitted to the logical operation circuit301 for performing subsequent processes such as fingerprint recognition.In some embodiments, the fingerprint sensing circuit 200 obtains thefingerprint grey levels according to the approaches of prior art, thatis, each fingerprint grey level is corresponding to one sensingelectrode instead of multiple sensing electrodes, but the logicaloperation circuit 301 would perform a method for processing thefingerprint image in order to improve the identification ability.

FIG. 4 is a schematic diagram illustrating the fingerprint imageaccording to an embodiment. Referring to FIG. 2 to FIG. 4, a fingerprintimage 400 includes fingerprint grey levels 410(1,1)-410(3,4), which arerespectively corresponding to the sensing units 210(1,1)-210(3,4). Inthe embodiment of FIG. 4, each fingerprint grey level is obtainedaccording to the voltage on one sensing electrode. The darkerfingerprint grey levels 410(1,1), 410(1,2), 410(2,2), 410(2,3), 410(3,3)and 410(3,4) represent valleys, and the other brighter fingerprint greylevels represent ridges. In FIG. 4, the grey level difference betweenridge and valley is small, and therefore it is hard to identify them.However, the logical operation circuit 301 would receive the fingerprintgrey levels 410(1,1)-410(3,4), and performs weighting summation one morethan one fingerprint grey levels (also referred to first fingerprintgrey levels) to obtain a new fingerprint grey level (also referred tofirst new fingerprint grey level) to replace one original fingerprintgrey level. The operation is written in a following equation (2).I′ _((i,j)∈C)=Σ_((m,n)∈C) w _(m,n) ×I _(m,n)  (2)

I′_((i,j)) denotes the new fingerprint grey level at the i^(th) columnand j^(th) row. C denotes a set including multiple coordinates, in whichthe coordinate (i,j) belongs to the set C. w_(m,n) is a real number.I_(m,n) denotes the fingerprint grey level at the m^(th) column and then^(th) row. All coordinates (m,n) belong to the set C. For example, theset C includes coordinates (1,1), (1,2), (2,1), (2,2), and (i,j)=(1,1).In other words, the weighting summation is performed on the firstfingerprint grey levels 410(1,1), 410(1,2), 410(2,1) and 410(2,2) toobtain a first new fingerprint grey level to replace the originalfingerprint grey level 410(1,1). In some embodiments, all the weightsw_(m,n) are ¼, and therefore the example above calculates the mean ofthe first fingerprint grey levels 410(1,1), 410(1,2), 410(2,1) and410(2,2). However, in some embodiments, all the weights w_(m,n) may be1, or the weights w_(m,n) may be different from each other. For example,w_(1,1) may be relatively larger than and the other weights w_(1,2),w_(2,1), w_(2,2) due to its position. The values of the weights w_(m,n)are not limited in the invention.

In some embodiments, after calculating the new fingerprint grey level410(1,1), the logical operation circuit 301 may perform weightingsummation on the fingerprint grey levels 410(1,2), 410(1,3), 410(2,2)and 410(2,3) to obtain a new fingerprint grey level to replace thefingerprint grey level 410(1,2). From another aspect, the fingerprintgrey levels 410(1,2), 410(1,3), 410(2,2) and 410(2,3) may be referred tosecond fingerprint grey levels. Part of the second fingerprint greylevels is identical to part of the first fingerprint grey levels. Thatis to say, the fingerprint grey levels 410(1,2) and 410(2,2) arerepeatedly used.

In the aforementioned embodiment, the weighting SU ruination isperformed on 4 fingerprint grey levels to replace one originalfingerprint grey level, in which 2 fingerprint grey levels arerepeatedly used. But, in other embodiments, the weighting summation maybe performed on N fingerprint grey levels to replace one originalfingerprint grey level, in which M fingerprint grey levels arerepeatedly used. N is a positive integer greater than 1, and M is apositive integer less than N. For example, in the case of N=9 and M=6,the weighting summation may be performed on the fingerprint grey levels410(1,1), 410(1,2), 410(1,3), 410(2,1), 410(2,2), 410(2,3), 410(3,1),410(3,2) and 410(3,3) to replace the fingerprint grey level 410(2,2);and the weighting summation is performed on the fingerprint grey levels410(1,2), 410(1,3), 410(1,4), 410(2,2), 410(2,3), 410(2,4), 410(3,2),410(3,3) and 410(3,4) to replace the fingerprint grey level 410(2,3).

After the aforementioned operations are performed, a fingerprint imageis generated, as shown in FIG. 5, that includes fingerprint grey levels510(1,1)-510(3,4). The darker fingerprint grey levels 510(1,1),510(1,2), 510(2,2), 510(2,3), 510(3,3), 510(3,4) represent valleys, andthe other brighter fingerprint grey levels represent ridges. Compared toFIG. 4, the grey level difference in FIG. 5 is larger, and thus theidentification ability is improved.

FIG. 6 is a diagram illustrating a flowchart of a method for processinga fingerprint image. The method may be applied to the aforementionedfingerprint sensing circuit. Referring to FIG. 6, in step 601,fingerprint grey levels are received from the fingerprint sensingcircuit. In step S602, weighting summation is performed on firstfingerprint grey levels to obtain a first new fingerprint grey level toreplace one of the first fingerprint grey levels. In some embodiments,the method further includes step S603, in which the weighting summationis performed on second fingerprint grey levels to obtain a second newfingerprint grey level to replace one of the second fingerprint greylevels. In particular, part of the first fingerprint grey levels isidentical to part of the second fingerprint grey levels. However, allthe steps in FIG. 6 have been described in detail above, and thereforethe description will not be repeated.

Although the present invention has been described in considerable detailwith reference to certain embodiments thereof, other embodiments arepossible. Therefore, the spirit and scope of the appended claims shouldnot be limited to the description of the embodiments contained herein.It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without de parting from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims.

What is claimed is:
 1. A fingerprint sensing circuit, comprising: aplurality of sensing units, arranged as a plurality of columns and aplurality of rows, wherein each of the sensing units comprises a sensingelectrode and a transfer switch having a first terminal, a secondterminal and a control terminal, and the first terminal of the transferswitch is connected to a respective sensing electrode; a sensingcircuit, configured to be electrically connected to the sensingelectrodes in at least two first sensing units of the sensing units in afirst period to obtain a first fingerprint grey level; a plurality offirst conductive lines respectively corresponding to the rows, whereineach of the first conductive lines is electrically connected to thecontrol terminals of the transfer switches in one of the rows; aplurality of second conductive lines respectively corresponding to thecolumns and electrically connected to the sensing circuit, wherein eachof the second conductive lines is electrically connected to the secondterminals of the transfer switches in one of the columns; a plurality ofcolumn switches respectively disposed on the second conductive lines anddisposed between the sensing units and the sensing circuit; a row selectcircuit, configured to transmit a first signal to the first conductivelines corresponding to a first row and a second row in the first periodfor turning on the transfer switches in the first row and the secondrow; and a column select circuit, configured to turn on the columnswitches corresponding to a first column and a second column in thefirst period, wherein the row select circuit is further configured totransmit the first signal to the first conductive lines corresponding tothe first row and the second row in a second period to turn on thetransfer switches in the first row and the second row, wherein thecolumn select circuit is further configured to turn on the columnswitches corresponding to the second column and a third column in thesecond period.
 2. An electronic device, comprising: a fingerprintsensing circuit, comprising: a plurality of sensing units, arranged as aplurality of columns and a plurality of rows, wherein each of thesensing units comprising a sensing electrode and a transfer switchhaving a first terminal, a second terminal and a control terminal, andthe first terminal of the transfer switch is connected to a respectivesensing electrode; a sensing circuit, configured to be electricallyconnected to the sensing electrodes in at least two first sensing unitsof the sensing units in a first period to obtain a first fingerprintgrey level; a plurality of first conductive lines respectivelycorresponding to the rows, wherein each of the first conductive lines iselectrically connected to the control terminals of the transfer switchesin one of the rows; a plurality of second conductive lines respectivelycorresponding to the columns and electrically connected to the sensingcircuit, wherein each of the second conductive lines is electricallyconnected to the second terminals of the transfer switches in one of thecolumns; a plurality of column switches respectively disposed on thesecond conductive lines, and disposed between the sensing units and thesensing circuit; a row select circuit, configured to transmit a firstsignal to the first conductive lines corresponding to a first row and asecond row in the first period for turning on the transfer switches inthe first row and the second row; and a column select circuit,configured to turn on the column switches corresponding to a firstcolumn and a second column in the first period, wherein the row selectcircuit is further configured to transmit the first signal to the firstconductive lines corresponding to the first row and the second row in asecond period to turn on the transfer switches in the first row and thesecond row, wherein the column select circuit is further configured toturn on the column switches corresponding to the second column and athird column in the second period.